1. Field of the Invention
The present invention relates to high power microwave tubes, and, more particularly, to a method for fabricating a coating on circuit parts employed in such microwave tubes that introduces controlled RF losses in the circuit.
2. Description of Related Art
Extra loss is introduced in RF circuits of most coupled-cavity traveling-wave tubes (TWTs) for stability or for reduced gain variations with frequency. Loss is also introduced in selected cavities of klystron circuits for control of bandwidth and gain flatness.
Presently, loss is typically introduced in coupled-cavity RF circuits by means of lossy ceramic elements (called "loss buttons") or by coating cavity surfaces with KANTHAL.RTM. heating element alloy by flame spraying. (KANTHAL is a trademark of Kanthal Corp., Bethel, Conn.; the alloy is an iron-chromium-aluminum alloy.)
Loss buttons suffer from a variety of ills in different applications, such as (a) high material cost, particularly at lower frequencies (S and C band); (b) labor intensiveness, with buttons requiring individual tuning and/or a substantial effort in circuit matching; (c) limited power handling capacity, giving rise to power fade with increasing duty or suffering overheating and cracking; and (d) lot-to-lot variability in RF characteristics.
Applying KANTHAL alloy by flame spraying is a process that is difficult to control for consistency. The process is also not practical at mm wave frequencies, due to the use of thin and fragile circuit parts at such frequencies, with tight tolerances on dimensions; the process creates a coating that is too thick and coarse to maintain sufficient precision on the critical dimensions of the structure, and it may result in distortion of the parts.
Accordingly, it is desired to introduce loss in RF circuits by a reliable and reproducible method.